Method of producing metallic iron

ABSTRACT

In a method of producing metallic iron by reducing and melting a raw material, which contains an iron oxide-containing material and a carbonaceous reducing agent, by heating, the raw material for producing metallic iron is supplied after a powder and granular atmosphere control carbonaceous material is spread on a hearth of a heat-reducing furnace. In this case, a non-resolidificable carbonaceous material is used as the atmosphere control carbonaceous material, and thus a phenomenon that the powder and granular carbonaceous material is resolidified into a rice-cracker-like shape can be suppressed, thereby permitting solid-phase reduction with high efficiency and stable operationality.

TECHNICAL FIELD

The present invention relates to a method of producing metallic iron,and particularly to a method improved for permitting a smooth continuousoperation by preventing a phenomenon that in producing metallic iron byheat-reducing a mixture of an iron oxide-containing material and acarbonaceous reducing agent on a moving hearth, an atmosphere controlcarbonaceous material, which is spread on the hearth to increase anatmospheric reduction potential on the hearth for heating reduction, isresolidified into a sheet shape to inhibit operationality.

BACKGROUND ART

Relatively new methods of producing metallic iron by heat-reducing aniron oxide source such as an iron ore or the like include a method ofproducing metallic iron comprising reducing iron oxide by heating, on amoving hearth, a mixed power containing an iron oxide source such asiron ore and a carbonaceous reducing agent such as a carbonaceousmaterial, or a carbonaceous material-containing raw materialagglomerated by pelletizing the mixture.

In carrying out this method, a known method is performed, in which inorder to increase a reduction potential on the hearth forheating-reduction to improve a reduction efficiency, an atmospherecontrol carbonaceous material is spread on the hearth before the rawmaterial is charged (for example, Japanese Unexamined Patent ApplicationPublication Nos. 11-106816, 11-106816, 11-172312, 11-335712, 2000-45008,etc.). The atmosphere control carbonaceous material is confirmed toeffectively function to prevent direct contact between a hearthrefractory and metallic iron and generated slag, which are produced byheating-reduction, and to suppress corrosion of the hearth refractory.

As a result of the advance of research on a method of producing metalliciron by using an atmosphere control carbonaceous material, the inventorsfound that the above-described conventional methods have the unsolvedproblems below.

The greatest problem pointed out in the conventional methods is that apowder and granular carbonaceous material used for controlling anatmosphere is fused and solidified into a rice-cracker-like shape in thestep of heat-reducing the iron oxide-containing raw material to causewarping, depending on the type of the powder and granular carbonaceousmaterial used, thereby significantly hindering a continuous operation.When such a phenomenon occurs on the hearth during an operation, thefollowing various problems are caused.

(1) The metallic iron and generated slag produced by heating-reductionare generally solidified by cooling on the lowermost stream side of aproduction apparatus, and then discharged from the hearth by using ascraper device such as a screw or the like. However, the warpedcarbonaceous material resolidified into a rice-cracker-like shape iscaught by the scraper device, thereby significantly inhibiting dischargeof the metallic iron and generated slag from the hearth.

(2) When the carbonaceous material re-solidified into arice-cracker-like shape is forcedly discharged from the hearth by usingthe scraper device, a large load is applied to the scraper device tocause a failure in the device. Also, the hearth refractory is damaged bythe re-solidified carbonaceous material to significantly deterioratedurability.

(3) The metallic iron produced by reduction is partially contained inthe re-solidified carbonaceous material, thereby deteriorating arecovery ratio of the metallic iron.

(4) When a resolidificable carbonaceous material is spread on the hearthbefore a raw material is charged, the carbonaceous material isre-solidified into a rice-cracker-like shape to cause warping.Therefore, when a raw material is supplied on the re-solidifiedcarbonaceous material, the raw material flows toward a lower portion ordrops into a crack of the carbonaceous material layer, thereby failingto charge the raw material in a uniform thickness.

Furthermore, most of the carbonaceous material discharged from thehearth still has high reduction activity. However, in the conventionaltechnique, the carbonaceous material is discarded with substantially nofurther treatment, leaving room for improvement from the viewpoint ofeffective utilization of valuable resources.

The present invention have been achieved in consideration of theabove-described situation, and an object of the present invention is toresolve the above various problems due to the rice-cracker-likecarbonaceous material produced by re-solidification of a powder andgranular carbonaceous material used for controlling an atmosphere.Another object of the present invention is to establish a technique foreffectively recycling the used carbonaceous material still havingreduction activity as a variable resource, to decrease the consumptionof the atmosphere control carbonaceous material.

DISCLOSURE OF INVENTION

In order to achieve the objects, a method of producing metallic ironaccording to the present invention comprises heating, on a movinghearth, a raw material containing an iron oxide-containing material anda carbonaceous reducing agent to reduce iron oxide contained in the rawmaterial, wherein the raw material is supplied after a powder andgranular atmosphere control carbonaceous material is spread on thehearth, and a non-resolidificable carbonaceous material is used as theatmosphere control carbonaceous material.

As the atmosphere control carbonaceous material used in the presentinvention, a preferred carbonaceous material has a grain diameter ofsubstantially 3.35 mm or less, contains 20% by mass or more of grainshaving a grain diameter in the range of 0.5 to 3.35 mm, and has amaximum fluidity degree of 0 (zero). Another preferred example of theatmosphere control carbonaceous material is a non-resolidificablecarbonaceous material obtained by heat-treating a resolidificablecarbonaceous material at a temperature of about 500° C. or more.

Also, a recovered carbonaceous material having been heated because ofuse as the atmosphere control carbonaceous material in a metallic ironproducing apparatus loses its resolidificability due to the heattreatment, and is made non-resolidificable. Therefore, the recoveredcarbonaceous material can also be effectively used as thenon-resolidificable carbonaceous material, and a carbonaceous material,which is originally non-resolidificable, maintains itsnon-resolidificability under the heat treatment, and can thus berecovered and recycled.

In the present invention, another effective carbonaceous material is amixed non-resolidificable carbonaceous material containing aresolidificable carbonaceous material and a non-resolidificablecarbonaceous material. In this case, a flesh carbonaceous material canbe used as the resolidificable carbonaceous material, and a carbonaceousmaterial heat-treated at a temperature of about 500° C. or more can bepreferably used as the non-resolidificable carbonaceous material.Particularly, the carbonaceous material having been heated in themetallic iron producing apparatus loses its resolidificability due tothe heat treatment, and thus the carbonaceous material having beenheated is recovered and recycled to cause the advantage that theconsumption of the atmosphere control carbonaceous material can bedecreased in cooperation with the effective utilization of wastematerials.

Furthermore, by using the carbonaceous material recycling method,fine-grain metallic iron and generated slag, which are mixed in therecovered carbonaceous material, can be recovered by treatment in a nextstep, and thus the recovery ratio of the metallic iron can be increased.When the generated slag is effectively used as a by-product, therecovery ratio of the slag can also be increased.

In the use of the mixture of the resolidificable carbonaceous materialand the non-resolidificable carbonaceous material, a preferred mixingratio depends on the resolidification force of the resolidificablecarbonaceous material used, but the mixing ratio of thenon-resolidificable carbonaceous material is preferably in the range of50 to 90% by mass relative to 50 to 10% by mass of the resolidificablecarbonaceous material.

In carrying out this method, a portion of the metallic iron,particularly fine-grain metallic iron, discharged from the moving hearthfurnace is preferably returned to the moving hearth furnace, and thecarbonaceous material can be efficiently recovered by using staticelectricity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart of reduction and melting performed in an exampleof the present invention.

FIG. 2 is a flowchart of reduction and melting performed in anotherexample of the present invention.

FIG. 3 is a flowchart of reduction and melting performed in a furtherexample of the present invention.

FIG. 4 is a flowchart of reduction and melting preformed in a referenceexample of the present invention.

FIG. 5 is a flowchart showing a step of separating metallic iron,generated slag, and a recovered carbonaceous material according to thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention can be applied to the methods disclosed in, forexample, U.S. Pat. No. 6,036,744, and Japanese Unexamined PatentApplication Publication Nos. 9-256017, 2000-144224 and 11-131119, inwhich an iron oxide-containing material such as an iron ore is mixedwith a carbonaceous reducing agent such as a carbonaceous material, andif required, the resultant mixture is agglomerated or formed intopellets, reduced by heating on a moving hearth, and further heated tomelt and aggregate the produced reduced iron and separate the generatedslag, to produce granular or lump high-purity metallic iron.

As described above with respect to the related art, it is already knownthat in practical use of the above-described method of producingmetallic iron, as a means for efficiently progressing heating-reductionof the iron oxide source in the raw material mixture on the movinghearth, a powder and granular atmosphere control carbonaceous materialis spread on the hearth before the raw material is charged, to maintainthe reduction potential on the hearth at a high level duringheating-reduction. Consequently, the reduction efficiency is increasedto improve the recovery ratio of metallic iron.

However, the conventional technique causes the above-described varioustroubles in the operation due to the phenomenon that the carbonaceousmaterial is resolidified in a sheet shape by heat for reduction andmelting, depending on the type of the atmospheric control carbonaceousmaterial used.

Therefore, research was advanced for resolving the above-describedproblems due to the resolidification of the atmosphere controlcarbonaceous material, and for permitting efficient and smoothproduction of metallic iron from various angles. As a result, it wasfound that the above problems can be solved by using, as the atmospherecontrol carbonaceous material, a carbonaceous material, which canmaintain a powder and granular state without being resolidified evenunder a heating condition for reduction and melting of the raw materialmixture. This resulted in achievement of the present invention.

Therefore, the present invention is characterized by using anon-resolidificable carbonaceous material as the atmosphere controlcarbonaceous material. Examples of the non-resolidificable carbonaceousmaterial include the following.

(1) Carbonaceous material having a grain diameter of substantially 3.35mm or less, containing 20% by mass or more, preferably 40% by mass ormore, of grains having a grain diameter in the range of 0.5 to 3.35 mm,and having a maximum fluidity degree (which will be described below) ofzero (0):

The carbonaceous material satisfying such a grain size pattern andmaximum fluidity degree is not resolidified under a basically reducinghigh-temperature condition (generally 700 to 1600° C., and moregenerally 900 to 1500° C.), and maintains a powder and granular state.However, as described in the examples below, it was confirmed that evenwith a maximum fluidity degree of zero, a powder and granularcarbonaceous material having a grain diameter of 3.35 mm or less, andcontaining less than 20% by mass of coarse grains having a graindiameter in the range of 0.5 to 3.35 mm causes resolidification underthe temperature condition of reduction and melting. Although the reasonfor this is not known yet at present, it is considered that thecarbonaceous material contains a large amount of fine grains of lessthan 0.5 mm, and the fine grains function as a binder to promoteresolidification of the carbonaceous material. Furthermore, as the ratioof the fine grains of less than 0.5 mm increases, scattering due to anair flow in the metallic iron producing apparatus undesirably increases.

A coarse carbonaceous material having a grain diameter of over 3.35 mmcauses no problem due to resolidification, but mixing of the coarsematerial having a grain diameter of over 3.35 mm must be avoided as muchas possible because the function as the atmosphere control carbonaceousmaterial deteriorates due to a shortage of surface area. In order toeffectively exhibit the function as the atmosphere control carbonaceousmaterial, a material containing 60% by mass or less of the coarse grainsin the range of 0.5 to 3.35 mm is preferably used.

(2) Carbonaceous material heat-treated at a temperature of about 500° C.or more:

The inventors confirmed that a resolidificable carbonaceous materialloses its resolidificability by heat-treatment at a temperature of about500° C. or more in a non-oxidizing atmosphere, and is madenon-resolidificable. Therefore, when a resolidificable carbonaceousmaterial short of suitability is heat-treated at about 500° C. or more,preferably about 600 to 1200° C., for about 5 to 15 minutes in anon-oxidizing atmosphere, the material can be changed to a carbonaceousmaterial which can be used as the non-resolidificable carbonaceousmaterial without any problem.

In producing metallic iron by using the moving hearth furnace, therecovered carbonaceous material separated from the metallic iron and thegenerated slag and recovered after being used as the atmosphere controlcarbonaceous material experiences heating corresponding to the heattreatment, and is thus converted to a non-resolidificable carbonaceousmaterial by heating in the non-oxidizing atmosphere. Therefore, therecycled carbonaceous material can be effectively used as the atmospherecontrol carbonaceous material with grain size control after recoveryaccording to demand.

(3) Mixed non-resolidificable carbonaceous material containing aresolidificable carbonaceous material and a proper amount ofnon-resolidificable carbonaceous material:

As seen from the examples below, when an appropriate amount ofnon-resolidificable carbonaceous material is mixed with aresolidificable carbonaceous material having a maximum fluidity degreeof more than zero, the mixed carbonaceous material can be madenon-resolidificable as a whole, and can thus be used as thenon-resolidificable atmosphere control carbonaceous material. As thenon-resolidificable carbonaceous material, the above-descriednon-resolidificable carbonaceous material obtained by heat-treating aresolidificable carbonaceous material, and the above-described recoveredcarbonaceous material recovered after experiencing the heating in themetallic iron producing apparatus can be used.

The preferred amount of the non-resolidificable carbonaceous materialmixed with the resolidificable carbonaceous material depends upon thedegree of resolidificability of the resolidificable carbonaceousmaterial used, for example, the value of a maximum fluidity degree. Withthe resolidificable carbonaceous material originally having a lowfluidity degree, the material can be converted to a non-resolidificablematerial by mixing only a small amount of non-resolidificablecarbonaceous material, while with the resolidificable carbonaceousmaterial having a high fluidity degree, a relatively large amount ofnon-resolidificable carbonaceous material must be mixed. However, thestandard amount of the non-resolidificable carbonaceous material mixedfor making the resolidificable carbonaceous material non-resolidificableis in the range of 50 to 90% by mass relative to 50 to 10% by mass ofthe resolidificable carbonaceous material, more generally in the rangeof 40 to 90% by mass relative to 60 to 10% by mass of theresolidificable carbonaceous material.

Of the above-described materials, most preferable materials for thepresent invention include the recovered carbonaceous material modifiedto non-resolidificable by the heat treatment in the metallic ironproducing apparatus, and the mixed carbonaceous material madenon-resolidificable by mixing the recovered carbonaceous material withthe resolidificable carbonaceous material. In the conventional techniqueusing the atmosphere control carbonaceous material for improving thereduction efficiency, the atmosphere control carbonaceous materialdischarged together with metallic iron and generated slag is neitherrecovered nor recycled, but it is mostly discarded for reclamationtogether with the generated slag.

However, the carbonaceous material used as the atmosphere controlcarbonaceous material must maintain reduction activity for preventingreoxidation of metallic iron even in the final stage of heatingreduction and melting, and thus the discharged carbonaceous material hassignificant reduction activity, and can be used as a reducing agent.Furthermore, as described above, the recovered carbonaceous material isconverted to the non-resolidificable carbonaceous material by heat forreduction and melting of the iron oxide source. Therefore, byeffectively using the recovered non-resolidificable carbonaceousmaterial as a recycled carbonaceous material, resolidification of thecarbonaceous material can be securely prevented to further improveoperation stability, as compared with the use of a fresh carbonaceousmaterial.

Furthermore, when the recovered carbonaceous material is recycled asdescribed above, a significant amount of fine-grain metallic ironcontained in the recovered carbonaceous material is again returned tothe metallic iron producing apparatus, thereby contributing to animprovement in the recovery efficiency of the metallic iron. Similarly,when the generated slag is also recovered as a valuable resource, theslag mixed as fine grains in the recovered carbonaceous material is alsoreturned to the metallic iron producing apparatus together with thecarbonaceous material, thereby contributing to an improvement in therecovery efficiency of the generated slag.

By making good use of the present invention as described above, thefollowing many advantages can be obtained.

1) The problems of resolidification of the carbonaceous material can besolved.

2) The carbonaceous material, which maintains reduction activity andwhich is conventionally discarded, can be effectively used, therebycontributing to a decrease in the consumption of the carbonaceousmaterial.

3) Metallic iron fine grains which are discarded and lost together withthe carbonaceous material are recycled together with the carbonaceousmaterial, thereby improving the recovery efficiency of the metalliciron.

4) Similarly, the generated slag can be recovered as a valuableresource, improving the recovery efficiency.

Furthermore, the construction of an apparatus used for carrying out thepresent invention, i.e., a moving hearth-type heating furnace forreduction and melting, is not limited, and all reduction meltingfurnaces disclosed in, for example, U.S. Pat. No. 6,036,744, andJapanese Unexamined Patent Application Publication Nos. 9-256017,2000-144224 and 11-131119 can be used. However, as a preferredapparatus, a rotary hearth furnace is recommended for continuouslyefficiently performing an operation comprising heat-reducing a rawmaterial, melting reduced iron and aggregating the molten iron to agranular material, and separating the generated slag.

In the present invention, the type of the iron oxide-containing materialused as the iron source is not limited, and besides a typical iron ore,an iron making waste material and discard such as iron-making andsteel-making dust discharged from an iron-making factory, classified andrecovered iron scraps, and the like can be used as the raw material.These iron sources can be used in a combination of a plurality of thesources according to demand.

Also, the carbonaceous reducing agent necessary for reducing the ionoxide-containing material is not limited, and any material can be usedas long as it comprises carbon as a main component and releases reducingcarbon monoxide by combustion or pyrolysis. Furthermore, as theatmosphere control carbonaceous material, any material can be used aslong as it can be converted to a non-resolidificable carbonaceousmaterial by modifying or mixing with any one of various types of coal orcoke, which are adaptable to the objects of the present invention,according to demand.

The specified condition for reduction and melting is not particularlyspecific, and the conditions disclosed in, for example, U.S. Pat. No.6,036,744, and Japanese Unexamined Patent Application Publication Nos.9-256017, 2000-144244 and 11-131119 may be used. However, a preferredstandard condition is satisfied by a two-step heating system in whichsolid-phase reduction is mainly progressed at a furnace temperature keptat 1200 to 1500° C., preferably in the range of 1200 to 1400° C., andthen the furnace temperature is increased to 1400 to 1500° C. to reducethe remaining iron oxide, and to melt the produced metallic iron(reduced iron) to aggregate the iron into grains. By setting thiscondition, granular metallic iron can be stably produced in high yield.The necessary time is about 8 to 13 minutes. Under this condition,solid-phase reduction of iron oxide, melting and coalescence can becompleted within such a short time.

Furthermore, in the present invention, as described above, theatmosphere control carbonaceous material is spread on the hearth forreducing iron oxide to keep the reduction potential on the hearth at ahigh level, thereby stably securing a high reduction efficiency withoutreoxidization of the reduced iron, particularly at the final stage ofheating reduction or at the time of melting of the reduced iron. Thecarbonaceous material spread on the hearth refractory can also prevent aphenomenon that the molten iron and slag produced by reduction andmelting are brought into direct contact with the hearth refractory todeteriorate the refractory, thereby contributing to life lengthening ofthe hearth refractory. In order to effectively exhibit these functions,the thickness of the atmosphere control carbonaceous material spread onthe hearth surface is preferably in the range of 1 to 10 mm.

In general, the metallic iron, the generated slag, and the atmospherecontrol carbonaceous material are discharged in a mixed state from themetallic iron producing apparatus. Of these discharged materials, themetallic iron can be recovered by magnetic force, or the like. A portionof the metallic iron, particularly fine grains, is preferably returnedto the moving hearth furnace and re-aggregated therein, therebypreferably increasing the yield of coarse-grain metallic iron which caneasily be handled as a product and which causes less oxidativedeterioration. In addition, the generated slag and the atmospherecontrol carbonaceous material can be substantially separated by asieving operation, but both materials are preferably separated by usingstatic electricity because grains having substantially the same size orfine grains, which cannot be separated by a sieve, can easily beseparated. A combination of a separation operation using a sieve ormagnetic force and a separation operation using static electricity iseffective according to demand.

EXAMPLES

Although the construction of the present invention and the operationwill be described in detail below with reference to examples, thepresent invention is not limited to these examples, and can be carriedout according to appropriate modifications within the scope of the gistof the present invention described above and below. These modificationsare included in the technical field of the present invention.

Example 1

Each of carbonaceous materials having the chemical compositions shown inTable 1 below was independently subjected to the heating test describedbelow. The grain size of each carbonaceous material was controlled inthe range of 0.5 to 1.0 mm. Each carbonaceous material was heated at1000° C. for 90 seconds in a nitrogen atmosphere in a tubular electricfurnace, cooled and then observed with respect to the appearance toexamine the presence of resolidification. Also, the maximum fluiditydegree of each carbonaceous material was measured. The maximum fluiditydegree is defined by JIS M8801, and can be determined by using GieselerPlastometer. The maximum fluidity degree is a value represented by logDDPM.

The results are as shown in Table 1. Carbonaceous materials A to Fhaving a maximum fluidity degree of 0 (zero) did not exhibitresolidificability, and maintained a powder and granular state afterheat treatment. On the other hand, carbonaceous materials G to J havinga maximum fluidity degree of over 0 were resolidified in a bulk state inthe tube electric furnace. It was also confirmed that carbonaceousmaterials K and L obtained by heat-treating carbonaceous materials I andJ, respectively, at 1000° C. for 8 minutes in a nitrogen atmosphere weremade non-resolidificable by the heat treatment. TABLE 1 Melt softeningCarbonaceous Analytical value (mass %) maximum material Ash VolatileFixed Result of fluidity Symbol sample content content carbon Totalheating test degree A Carbonaceous 13.5 2.0 84.5 100 No re- 0 materialsolidification B Carbonaceous 0.1 7.5 92.7 100 No re- 0 materialsolidification C Carbonaceous 17.0 5.6 77.1 100 No re- 0 materialsolidification D Carbonaceous 4.5 7.1 88.5 100 No re- 0 materialsolidification E Carbonaceous 13.6 9.4 77.0 100 No re- 0 materialsolidification F Carbonaceous 16.7 16.9 66.4 100 No re- 0 materialsolidification G Carbonaceous 11.9 37.2 50.9 100 Re- 0.2 materialsolidification H Carbonaceous 9.8 15.9 74.3 100 Re- 0.5 materialsolidification I Carbonaceous 7.4 35.4 57.2 100 Re- 1.1 materialsolidification J Carbonaceous 8.8 19.6 71.6 100 Re- 2.6 materialsolidification K Heat-treated No re- product of I solidification LHeat-treated No re- product of J solidification

Of the carbonaceous materials shown in Table 1, each of the carbonaceousmaterials exhibiting resolidificability was mixed with anon-resolidificable material, and the resultant mixture was heated at1000° C. for 90 seconds in a nitrogen atmosphere to examineresolidificability. The results are shown in Table 2. Table 2 indicatesthat a mixed non-resolidificable carbonaceous material can be obtainedby mixing an appropriate amount of non-resolidificable carbonaceousmaterial with a resolidificable carbonaceous material. In this case, itis recognized that with a resolidificable carbonaceous material having ahigh maximum fluidity degree, the mixing ratio of thenon-resolidificable carbonaceous material for removingresolidificability must be increased. TABLE 2 Mixing ratio of sub- Maincarbonaceous Sub-carbonaceous carbonaceous material material material(non- for making non- (resolidificable resolidificable resolidificablecarbonaceous material) carbonaceous material) (mass %) I K 80 J L 90 I C80 J C 90 H A 60 H B 60 H C 60 H D 60 H E 60 H F 60 G C 60

Example 2

In producing metallic iron by reducing and melting carbonaceousmaterial-containing iron ore pellets (grain diameter: 16 to 20 mm) by arotary hearth-type reduction melting apparatus using carbonaceousmaterial H (grain diameter: 3 mm or less) shown in Table 1 as anatmosphere control carbonaceous material according to the flowchart ofFIG. 1, experiment on a recycle of the atmosphere control carbonaceousmaterial was carried out. Namely, the atmosphere control carbonaceousmaterial (mixture of fresh material of carbonaceous material H and arecycled material thereof) was spread to a thickness of about 3 to 6 mmon a hearth of a raw material feed portion of the rotary hearth furnace,and raw material pellets were supplied onto the hearth under heating toreduce and melt the raw material pellets. Then, the produced reducediron and generated slag were cooled together with the atmosphere controlcarbonaceous material remaining on the hearth, and discharged from thehearth by a scraper device. The discharged material was put in amagnetic separator and through a sieve to separate the reduced ion, thegenerated slag and the remaining carbonaceous material. The separatedremaining carbonaceous material was recovered, returned as a recycledcarbonaceous material to the raw material feed portion, and then againused. The operation conditions for reduction and melting were asfollows.

[Operation Conditions]

Raw material pellet: An iron ore raw material having the compositionbelow was mixed with a carbonaceous material powder at a ratio by massof 78:22, and a small amount of binder was added to the resultantmixture. The mixture was then granulated and dried to obtain granularpellets having an average gain diameter of 18 mm.

-   -   Composition of iron ore raw material (% by mass): T. Fe; 68.1%,        SiO₂: 1.4%, Al₂O₃; 0.5%

Operation Conditions:

-   -   Heat-reduction zone; temperature . . . about 1350° C., retention        time . . . 10 minutes    -   Melting zone; temperature . . . about 1450° C., retention time .        . . 5 minutes.

A continuous operation was performed by this method using a mixture of40 parts by mass of fresh carbonaceous material and 60 parts by mass ofrecycled carbonaceous material. As a result, the mixed carbonaceousmaterial was not resolidified in the reduction melting step, and thusdischarge from the hearth furnace by the scraper device and recyclingcould be smoothly performed, thereby permitting a continuous operationwithout any problem.

Example 3

Another experiment was conducted according to the flowchart of FIG. 2using the same rotary hearth furnace-type reduction melting apparatus asdescribed above. In this apparatus, a fresh material of carbonaceousmaterial I (resolidificable) shown in Table 1, a fresh material ofcarbonaceous material F (non-resolidificable) shown in Table 1, and arecycled material recovered after being heated in the apparatus weremixed at a ratio by part of 20:20:60, and the resultant mixture was usedin a similar continuous operation. The raw material pellets used andoperation conditions were the same as Example 1.

As a result, the atmosphere control carbonaceous material was notresolidified at the discharge position of the cold-solidified productafter reduction and melting, thereby permitting smooth discharge of theproduct by the scraper device. Also, the discharged product was put in amagnetic separator and through a sieve to recover the granular metalliciron and to separate the generated slag, obtaining the residualcarbonaceous material. The recovered residual carbonaceous material(grain diameter: 3 mm or less) could be repeatedly used as thenon-resolidificable carbonaceous material without any problem.

Example 4

The degree of crushing of carbonaceous material F shown in Table 1 waschanged to prepare two types of carbonaceous materials respectivelyhaving the grain size patterns shown in Table 3, and each of the twocarbonaceous materials was used in the same heating test as Example 1 tocompare the presence of resolidification. The results are as shown inTable 3. Even with carbonaceous materials having the same composition,resolidificability depends upon the grain size pattern, and thecarbonaceous material containing 20% by mass or more of grains having agrain size in the range of 0.5 to 3.25 mm is not resolidified, while thecarbonaceous material containing less than 20% by mass of grains havinga grain size in the same range (i.e., containing over 80% by mass offine grains of less than 0.5 mm) is slightly resolidified. It is thusfound that proper control of the grain size pattern of a carbonaceousmaterial is also effective in preventing resolidification. TABLE 1 Gainsize distribution (mass %) Over 0.5 to Less than Result of 3.35 mm 3.35mm 0.5 mm Total heating test Carbonaceous 0.0 23.6 76.4 100 No re-material F-1 solidification Carbonaceous 0.0 18.2 81.8 100 Low re-material F-2 solidification

Example 5

50 g of carbonaceous material C shown in Table 1 was spread on arefractory tray of an experimental heating furnace, and about 170 g ofdry pellets (grain diameter: 9.5 to 13.2 mm) having the same rawmaterial composition as Example 1 was charged in a layer on carbonaceousmaterial C. Then, reduction and melting were performed at a furnacetemperature of 1450° C. for 20 minutes in a nitrogen atmosphere toproduce granular iron and generated slag. The grain size distributionsof the produced iron and generated slag were examined (Experiments 1 and2).

Also, reduction and melting were performed by the same method asdescribed above except that a mixture of 50 g of the same carbonaceousmaterial, 20 g of granular iron having a grain diameter of 1 to 3.35 mm,and 1 g of slag was spread on the refractory tray to produce granularion and generated slag. The grain size distributions of the producediron and generated slag were examined (Experiments 3 and 4). The resultsare shown in Table 4. TABLE 4 Experiment No. 1 2 3 4 Specimen Dry pelletGrain diameter (mm) 9.5-13.2 9.5-13.2  9.5-13.2  9.5-13.2 Weight (g) 173170 167 171 Recycled granular iron Grain diameter (mm) — —   1-3.35  1-3.35 Weight (g) — — 20 20 Recycled slag Grain diameter (mm) — —  1-3.35   1-3.35 Weight (g) — — 1 1 Weight of product after testGranular iron 6.7 mm or more (g) 24.7 28.5 23.4 25.4 3.35 to 6.7 mm (g)34.1 27.9 39.5 38.9 1 to 3.35 m (g) 22.2 23.5 34.3 35.0 Sub-total (g)81.0 79.9 97.2 99.3 Slag 6.7 mm or more (g) 0.1 0.0 0.0 0.2 3.35 to 6.7mm (g) 3.2 2.9 3.3 3.6 1 to 3.35 mm (g) 0.7 0.5 1.2 1.2 Sub-total (g)4.0 3.4 4.5 5.0 Total (g) 85.0 83.3 101.7 104.3

These experiments were carried out for confirming the degree of recoveryof the granular iron and slag mixed in the recycled carbonaceousmaterial in recycling the carbonaceous material used as the atmospherecontrol carbonaceous material. Experiments 1 and 2 are experimentalexamples on the assumption that the carbonaceous material is notrecycled, and Experiments 3 and 4 are experimental examples on theassumption that the carbonaceous material is recycled.

A comparison between Experiments 1 and 2 and Experiments 3 and 4 shownin Table 4 indicates that in Experiments 3 and 4 on the assumption thatthe carbonaceous material is recycled, the amounts of the producedgranular iron and slag having a diameter of 1 to 3.35 mm are decreasedas compared with the totals of the amounts in Experiments 1 and 2 andthe amounts of the granular iron and slag initially mixed with thecarbonaceous material, while the amounts of the produced products havinga diameter of 3.35 to 6.7 mm are accordingly increased. It is thus foundthat the granular iron and slag previously mixed in the carbonaceousmaterial (corresponding to the recycled carbonaceous material containinggranular iron and slag) coalesce in the reduction and melting process.

Example 6

As shown in FIG. 3, non-resolidificable carbonaceous material A (40parts by mass) and recycled carbonaceous material (60 parts by mass)were mixed to prepare a mixed carbonaceous material (100 parts by mass).The thus-prepared mixed carbonaceous material was spread on a hearth ofa rotary hearth furnace by the same method as Example 1, andcarbonaceous material-containing dry pellets were charged on the mixedcarbonaceous material. Then, reduction and melting were performed, andthe obtained product was cooled, discharged, and then sieved to recovergranular iron and generated slag having a grain diameter of about 3 mmor more suitable for industrial utilization. Consequently, acarbonaceous material of about 3 mm or less containing fine granulariron and slag was recovered. Therefore, the whole amount of therecovered carbonaceous material was cyclically used as a recycledcarbonaceous material, and 40 parts by mass of fresh carbonaceousmaterial was added to the recycled material to balance the productionline as a whole, permitting a smooth continuous operation.

Reference Example

Reduction and melting were performed by the same method as describedabove except that dry pellets and non-resolidificable carbonaceousmaterial A were used without a recycle of the carbonaceous materialaccording to the flowchart of FIG. 4. The produced granular iron andgenerated slag and the recovered carbonaceous material were sieved witha grain diameter of about 3 mm. In this case, about 9% by mass of thewhole metallic iron discharged from the furnace was contained as finegrain iron in the recovered carbonaceous material, causing a productloss corresponding to the fine grain iron. Similarly, in a case in whichthe carbonaceous material is not recycled, about 70% by mass of thewhole generated slag discharged from the furnace is discharged as finegrain slag together with the recovered carbonaceous material, therebycausing a loss corresponding to the discharged fine grain slag inrecovering the slag as a valuable resource.

Example 7

In each of the separating operations of Example 1 (FIG. 1) and Example2(FIG. 2), a mixture of the metallic iron and generated slag dischargedfrom the furnace, and the atmosphere control carbonaceous material wassubjected to magnetic separation to recover metallic iron as shown inFIG. 5. Then, a mixture of the remaining generated slag and atmospherecontrol carbonaceous material was triboelectrically charged, and thensupplied to an electrostatic separator provided with positive andnegative electrodes to separate the generated slag (negatively charged)and the atmosphere control carbonaceous material (positively charged).The separated atmosphere control carbonaceous material can be recycledin the same manner as shown in FIGS. 1 and 2.

As the electrical charging method, electrical charging methods otherthan the triboelectrical charging method, for example, an electricalcharging method using an ion generator, a corona electrical chargingmethod, and the like may be used.

Industrial Applicability

In the present invention having the above-descried construction, a rawmaterial containing an iron oxide-containing material and a carbonaceousreducing agent is heated on a moving hearth to reduce iron oxide in theraw material, to produce metallic iron. In the production, the rawmaterial is supplied after a powder and granular atmosphere controlcarbonaceous material is spread on the hearth. When anon-resolidificable carbonaceous material is used as the atmospherecontrol carbonaceous material, the carbonaceous material can beprevented from being resolidified to a rice-cracker-like shape causing afailure in discharge, thereby permitting a smooth continuous operationand suppressing damage to a hearth refractory to lengthen the lifethereof.

Furthermore, by using a method of recycling the carbonaceous materialrecovered from the metallic iron producing apparatus and using therecycled carbonaceous material as the atmosphere control carbonaceousmaterial, the consumption of the carbonaceous material can besignificantly decreased, and metallic iron and generated slag containedin the recovered carbonaceous material, which are conventionallydiscarded, can be recovered, thereby improving the recovery ratio.Therefore, a kill-two-birds-with-one-stone additional functional effectcan be obtained.

1. A method of producing metallic iron comprising: heating a rawmaterial, which contains an iron oxide-containing material and acarbonaceous reducing agent, on a moving hearth to reduce iron oxide inthe raw material, wherein the raw material is supplied after a powderand granular atmosphere control carbonaceous material is spread on thehearth, and a non-resolidificable carbonaceous material is used as theatmosphere control carbonaceous material.
 2. A producing methodaccording to claim 1, wherein a carbonaceous material having a graindiameter of substantially 3.35 mm or less, containing 20% by mass ormore of grains having a grain diameter in the range of 0.5 to 3.35 mm,and having a maximum fluidity degree of 0 (zero) is used as theatmosphere control carbonaceous material.
 3. A producing methodaccording to claim 1, wherein a heat-treated product of aresolidificable carbonaceous material is used as the atmosphere controlcarbonaceous material.
 4. A producing method according to claim 1,wherein a recovered carbonaceous material having been heated in ametallic iron producing apparatus is used as the atmosphere controlcarbonaceous material.
 5. A producing method according to claim 1,wherein a mixture of a non-resolidificable carbonaceous material and therecovered carbonaceous material having been heated in the metallic ironproducing apparatus is used as the atmosphere control carbonaceousmaterial.
 6. A producing method according to claim 1, wherein a mixednon-resolidificable carbonaceous material formed by mixing anon-resolidificable carbonaceous material with a resolidificablecarbonaceous material is used as the atmosphere control carbonaceousmaterial.
 7. A producing method according to claim 6, wherein a freshcarbonaceous material is used as the resolidificable carbonaceousmaterial, and a heat-treated carbonaceous material is used as thenon-resolidificable carbonaceous material.
 8. A producing methodaccording to claim 7, wherein a recovered carbonaceous material havingbeen heated in a metallic iron making apparatus is used as theheat-treated carbonaceous material.
 9. A producing method accordingclaim 5, wherein the mixing ratio of the non-resolidificablecarbonaceous material is 50 to 90% by mass relative to 50 to 10% by massof the resolidificable carbonaceous material.
 10. A producing methodaccording to claim 1, wherein a portion of the metallic iron dischargedfrom the moving hearth furnace is returned to the moving hearth furnace.11. A producing method according to claim 4, wherein the carbonaceousmaterial is recovered by using static electricity.